Generally, the topsides are supported by a support structure which is partially submerged. These support structures may rest on the seabed. The support structure extends to a certain height above the water level to support the topside at a height where it is safe from the impact of waves. So called “jackets” are a commonly used type of support structure. A jacket is basically a large steel structure comprising many beams, including legs and cross-beams which interconnect the legs. Other kinds of support structures are also widely used, such as structures made from concrete columns.
The offshore platforms typically have a life expectancy of about 30 years, but shorter and longer life spans are also known. At the end of their economic life span the offshore platform generally need to be decommissioned, which includes a removal of both the topside and the support structure. The removal operation may be carried out in separate operations. Generally the topside is removed first. Subsequently, the support structure is removed.
One method of removing a topside is to remove it in small pieces. This is a very time consuming process and therefore disadvantageous.
Another known method of lifting and removing a topside from its support structure is to use a heavy lift vessel comprising cranes. With the cranes, the topside is hoisted from the support structure and subsequently placed on the deck of the heavy lift vessel, a barge or other kind of transport vessel. This method works but has some disadvantages.
One disadvantage is that a very heavy lift vessel (or heavy crane vessel) with large capacity cranes is required. These may not always be available. Moreover, very large topsides may be too heavy for this method. Another disadvantage is that structural reinforcements or (re-)installation of removal lift points of the topside may be required to allow the hoisting process.
Another known method is to remove the topside in relatively large modules which are lifted by a heavy crane vessel. This method works but is also time consuming and requires a substantial amount of offshore work. The topside may not always be suitable to be divided in modules.
Various other kinds of specialized lifting vessels have been conceived, such as a so-called shear-leg crane barge, the “Pieter Schelte” by Allseas, the “Twin Marine Lifter”, the “Versatruss”, the “MPU Heavy Lifter”, and the “GM lift concept” by Prosafe. Some of these vessels have been built, others still remain in the design stage or have been cancelled during the design stage. These vessels have a common disadvantage in that the lifting vessel is quite complex and therefore quite expensive to build and operate. Some of these vessels have an additional disadvantage that the lifting capacity is limited.
Another known method is the so-called reversed float-over method. This method is a reversal of the float-over installation method. A specialized lift vessel such as a purposively built barge is positioned at least partially underneath the topside. The lift vessel comprises ballast tanks. With the lift vessel, the topside is lifted from the support structure by deballasting the ballast tanks.
This method also works, but has a disadvantage in that large horizontal loads are introduced into the support structure. This can be explained as follows. It takes a considerable period of time to transfer the total load of the topside with the deballasting process from the support structure to the lifting vessel. This stage of the operation may last several hours i.e. between 1 and 10 hours. During this time period, the combination of barge, topside and support structure is subject to vertical and horizontal loads as a result of wind, waves and current. These loads will be transferred from the barge onto the support structure via the topside. The vertical load generally does not pose a problem, but the support structure is generally not designed to take the horizontal loads. This applies in particular to dynamic loads. A risk of damage or even failure of the support structure may occur. Obviously, this is undesirable.